Accurate arrow launching capability of a typical compound bow is dependent upon five mechanically adjustable variables: upper and lower tiller distance (which is an expression of eccentric wheel timing), location of arrow on the bow string and location of arrow rest vertically (which relates to vertical arrow alignment), and location of arrow rest horizontally (which relates to horizontal arrow alignment). Error in any one or all of these variables will cause deviation in arrow flight.
Because of the five mechanically adjustable variables, bow manufacturers can not specify precisely where an arrow should locate on the string, nor how far an arrow should be positioned away from the riser. The only reference manufacturers supply with a new bow is for the customer to measure and write down the upper and lower tiller measurements. The customer is not informed that these measurements are the manufacturer's measurement of eccentric wheel timing.
When a bow manufacturer sells a bow, either the customer, or an archery shop, adds the accessories to a bow. Accessories include arrow rests, overdraws, arrows, string nocks, etc. Because of the five mechanically adjustable variables, tuning for accurate arrow flight frequently requires in excess of eight hours and occassionally in excess of forty hours. Some archers are never able to tune a bow for accurate arrow flight. Even an archery shop cannot identically tune two identical bows with identical accessories if the accessories are not mounted in precisely the same positions. Even after a bow is accessorized and tuned, retuning is required at regular intervals to compensate for string and cable stretch. Changing any of the five mechanically adjustable variables, or even arrow diameter, requires complete retuning.